On window resize qt5 only redraws changed parts of widgets.
If resize is minor (i.e. height has increased by 1 pixel, for example),
qt5 may consider most parts of widget not changed and skip redrawing them
and redraw only certain widget elements.

But if cairo is used for drawing, on resize previously drawed image of widget
is discarded and new one of different size is created.
New image is empty, but qt5 doesn't issue redraw for whole widget.
To mitigate this issue, data from old image of widget should be copied over
to image of new widget, qt5 will redraw it partially or fully if necessary.

Change-Id: Id950074efece9072bbfc002dfcb6ead813d5aeff
Reviewed-on: https://gerrit.libreoffice.org/62698Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>
Tested-by: Jenkins